It is becoming apparent that, in future, battery systems will be increasingly used both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet the requirements in respect of voltage and available power specified for a respective application, a large number of battery cells are connected in series. Since the current provided by a battery of this kind has to flow through all battery cells and a battery cell can conduct only a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current. This can be done either by providing a plurality of cell windings within a battery cell housing or by externally interconnecting battery cells. However, the problem of compensation currents possibly occurring between the battery cells which are connected in parallel on account of cell capacitances and voltages which are not exactly identical is encountered in this case.
The basic circuit diagram of a conventional electrical drive unit, as used, for example, in electric and hybrid vehicles or else in stationary applications, such as for rotor blade adjustment of wind power installations, is illustrated in FIG. 1. A battery 10 is connected to a DC voltage intermediate circuit which is buffered by an intermediate circuit capacitor 11. A pulse-controlled inverter 12 is connected to the DC voltage intermediate circuit, said pulse-controlled inverter providing sinusoidal currents, of which the phases are offset in relation to one another, by means of in each case two switchable semiconductor valves and two diodes at three taps 14-1, 14-2, 14-3 for operation of an electric drive motor 13. The capacitance of the intermediate circuit capacitor 11 has to be large enough to stabilize the voltage in the DC voltage intermediate circuit for a period of time over which one of the switchable semiconductor valves is on. A high capacitance in the mF range is produced in a practical application, such as an electric vehicle.
One disadvantage of the arrangement illustrated in FIG. 1 is that the weakest battery cell in the battery 10 determines the range, and that the defect in an individual battery cell already leads to failure of the entire vehicle. In addition, the modulation of the high voltages in the pulse-controlled inverter 12 leads to high switching losses and—since insulated gate bipolar transistor (IGBT) switches typically have to be used owing to the high voltages—likewise to high conduction losses.
A further disadvantage is that the same current flows through battery cells or modules contained in the system and therefore said battery cells or modules cannot be actuated individually. Therefore, there is no way to influence various states of individual battery cells.